Variable inductance signal translating device



July 5, 1949. H. s: HUFF 2,475,185

VARIABLE INDUCTANCE SIGNAL TRNSLATING DEV ICE Filed Oct. 31. 14e

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OUTPUT Cmorneg Patented July 5, 1949 VARIABLE vINDUC'I'ANCE SIGNAL TRANSLATING DEVICE Henry S. Huff, Princeton Junction, N. J assigncr to Radio Corporation of America, a corporation of Delaware Application Dctober 31, 1946, Serial No. 706,961

Claims. 1

This invention relates to electro-mechanical signal translating devices, and more particularly to an electric pickup device for use in the repro duction of sound from phonograph records.

Various pickups have been devised hereto-fore which employ a reactance element, usually a pacitor, to control the frequency or the amplitude of a radio frequency signal as a function of mechanical displacement. The radio frequency signal is detected or otherwise converted to an audio frequency signal for utilization. Such pickups employing capacitors have the disadvantage of being non-linear, owing to the fact that a capacitor comprising a moving plate and a fixed plate varies inversely as the distance between the plates. This'causes (1) generation of harmonics, which is undesirable, and (2) low frequency modulation with warped or off-center records, which iseven more undesirable.

Push-pull capacitor type pickups using two fixed plates and a movable plate between them exhibit less non-linearity but require more complicated circuit arrangements and are more difficult to manufacture. Close spacings are required to achieve reasonably high sensitivity, but any stray capacitance between. the stator plates tends to swamp out the desired capacitance variations.

The primary object of my present invention is to provide an improved pickup device which, while making use of reactance elements, will not be subject to the aforementioned and other disadvantages which characterize prior art pickups.

More particularly, it is an object of my present invention to provide an improved phonograph pickup which will exhibit a substantially linear output versus displacement characteristic, which will not generate harmonics, and which will not produce low frequency modulation even with badly warped or badly off-center records.

Another object of my present invention is to provide an improved pickup device as aforesaid which may be used either in a single ended circuit or in a push-pull circuit.

Still another object of my present invention is to provide an improved pickup device as above set forth which will have good sensitivity over a relatively wide range.

A further object of my present invention is to provide an improved vibration pickup device which is particularly useful in the reproduction of phonograph .records and the moving parts of which will be of minimum mass.

In accordance with my present invention, I make use of a gpluralitycf varia'ble-inductances instead of the customary capacitances. In one form of pickup device, I employ a plurality of inductor elements at least two of which are stationary and a third one of which is mounted for vibratory movement relative to the two stationary elements in response to actuating forces applied thereto, as by the phonograph record. The inductors may be either in the form of coils or they may be straight conductors. In any case, the three inductors are connected in series and are arranged in mutually inductive relation, the movable inductor being so related to the two stationary inductors that, upon vibratory movement thereof, it will more or less approach one of the stationary inductors while simultaneously receding from the other stationary inductor, and vice versa. The inductors are so connected that, when the vibratory inductor is thus moved toward one of the stationary inductors, the mutual inductance between it and said one stationary inductor will be decreased while the mutual inductance between it and the second of the two stationary inductors will be simultaneously increased, and when the movable inductor moves in the opposite direction, the same effects will be produced but in the opposite sense. The output of my improved inductor pickup may be applied to an oscillator which operates at a frequency of around 300 to 400 megacycles per second. For example, a tuned plate, tuned grid oscillator may be employed, the output of my improved pickup producing variations of the impedance in the grid circuit which, in turn, varies the plate current of the oscillator to provide, in a load resistor, a variable voltage drop which conforms to the motion of the movable inductor of the pickup.

The novel features of my invention, as well as additional objects and advantages thereof, will better be understood from the following description of one embodiment thereof, when read in connection with the accompanying drawing in which Figure 1 is a diagrammatic view showing an arrangement of coil inductors in accordance with my present invention and illustrating the principle involved in a pickup device according to this present invention,

Figure 2 is a similar view but showing an ar rangement of straight inductors,

Figure 3 is a view, partly in section and partly in elevation, of a pickup device constructed in accordance with my present invention and uti lizing straightsindnctors of the type illustrated in.-Figure2,-.

3 "cure t is a sectional view taken on the line IV of Figure 3, igure is a sectional view taken on the line "a V of Figure 3,

Figure 6 is an enlarged, exploded, perspective view of the inductor elements of Figure 3 and the phonograph needle holder to which the DIOV- able inductor is connected, and

Figure '7 is a wiring diagram of one form of reproducing circuit with which my improved pickup device may be used.

Referring more particularly to the drawing, wherein similar reference numerals indicate cor r sponding parts throughout, there are shown Figure 1, three inductors I, 2 and 3 the it, of coils or windings which are serially con" may be of the same self inductance L. Couplli between the coils I and 2 causes a math. ductance M1 which is additive with res the self inductances. Coupling between 3 and 3 produces a mutual inductance 31. bucks the self inductances because the two and 3 are connected in bucking relation. larly, coupling between the coils I and a mutual inductance M3 which bucks the ductances. The total inductance in the oil? is thus Let it be assumed, now, that the center coil 22 moves to the right with respect to the coils i and 3, as viewed in Figure 1. This increases the mutual inductance M2 and simultaneously decreases the mutual inductance M1 by substantially the same amount, thereby decreasing the total inductance LT by twice the value or the change in either of the mutual inductances. Motion of the coil 2 to the left, as viewed in Figure 1, will im 7 crease the total inductance LT in like man thus having an opposite effect, that is, a sim ar eifect in the opposite sense. It is noteworthy that the percentage change of inductance is increased, rather than decreased, by the presence of mutual inductance M3, which is principally a function of the closeness between the coils i and This is in sharp contrast to the balanced capacitor pickup where coupling between the stator plates decreases the percentage change of capacit rice.

Although inductors in the form of c or windings may be used as illustrated in Figure l (and even with advantage under some circuinstances), straight, parallel conductors may used in accordance with my present invention v illustrated in Figure 2. Here, the straight corn ductors I, 2 and 3 are again connected in series and are disposed in physically spaced, substantially parallel relation. The conductors i,

3 are, of course, equivalent to the coils i, 2

ll of Figure 1. Motion of the conductor 2 ct ure 2 to the right increases the total induc LT, and motion of the conductor 2 to the left 1 creases the total inductance. Again, coupling between the conductors I and 3 decreases the total inductance and therefore increases the percentage change of inductance by motion or the conductor 2 in either direction.

In Figures 3 to 6, inclusive, there is shown a phonograph pickup device constructed in accordance with my present invention and utilizing, as the inductor elements, three straight conduc tors such as illustrated in Figure 2. The conductors l and 3, which are the stator inductors, may be formed by a single wire '5 of about 0.025 inch in diameter bent into the form shown in Figure 6 and with the inductor portions l and 3 spaced apart about 0.035 inch on their centers, which leaves a space between them of about 0.01 inch. To provide the parallel inductor elements I and 3, the wire 5 may first be bent into a U-shape. The connecting portion 1 of the U is then bent upwardly for a purpose to be presently set forth. The free end portions 9 and H are also bent upwardly in parallel relation to the portion '2, after which the portion 9 is again bent forwardly to provide a terminal portion [3 while the portion 1 i is bent forwardly and then downwardly to provide a terminal portion IS.

The wire 5 is mounted on a block ll of insulating material which is provided with a bore i9 formed with a shoulder 2| beyond which the bore l9 continues on through the block H on a reduced diameter, as clearly shown in Figure 3. The block l! is also provided with a pair of parallel bores 23 and 25. To mount the wire member 5 on the block [1, the upwardly bent U-portion l is inserted into the bore l9 and locked therein by means of a pin or the like 26, while the end portions 9 and II are inserted through the bores 23 and 25, respectively, after which they are bent to provide the terminal portions 13 and i5.

Within the bore I9 is a flexible, vibratory stylus bar 21 formed with the plate 29 which bears against the shoulder 2| and which is formed therebeyond with a threaded shank 3! on which a nut 33 is received for locking the stylus bar 21 in place. At its free end, the stylus bar 2'5 is formed with a loop or socket 35 for the reception of a stylus or phonograph needle 3"! of sapphire or any other suitable material.

The movable conductor 2, which may be considered as an armature inductor, is made of bronze wire of about 0.005 inch in diameter and is connected at one end to the stylus bar 2; and at its other end to a spring anchoring plate of conductive material which is secured to the block [1 by means of screws 4! or the like. A set screw 43 which passes through the anchoring plate 39 and is threaded into the block H, as best seen in Figure 3, may be adjusted to apply more or less tension to the tenuous, wire inductor element 2. As clearly seen from Figures 4: and 6, the inductor 2 is disposed between and substantially parallel to the inductor elements 5 and 3, the three inductor elements being disposed in laterally spaced relation. The terminal portion 55 may be soldered or otherwise suitably connected to the conductive anchoring plate 39 whereby inductor 2 is connected at one end to the inductor I. In this way, the inductors I, 2 and 3 are all connected in series.

The pickup head described above may be mounted on a suitable pickup arm 45 by means of screws 47. A cover 49 may be placed around the pickup head and secured to the arm it by screws 5|. A conductor 53 is connected to the terminal portion I3 of the inductor elements and a second conductor 55 is connected to the conductive stylus bar IS, the conductor 55 being gr0unded as shown in Figure 3. If desired, the pickup arm 45 may be grounded and the conductor connected thereto.

A pickup device such as described above may be used with a circuit of the type shown in Figure '7 or with any other suitable circuit. In Figure 7, there is shown a tuned plate, tuned grid oscillator 51 which may be arranged to oscillate at a frequency from 300 to 400 megacycles per second. The pickup device is connected to the grid of the oscillator 51 by means of a transmission line which may be constituted by the pickup arm 55 and the conductors 53 and 55. In one pickup constructed according to the form of my invention shown in Figures 3 to 6, the total inductance was of the order of 0.01 microhenry, and the maximum inductance variation was onli a fraction of this. Since a pickup device of this sort has a very low impedance, the impedance is stepped up by the coaxial line 25, 53, 55 which operates as a transformer. By proper design of this line (that is, the surge impedance and length) any required impedance level may be obtained. The line section is preferably of about one quarter wavelength at the operating fre quency and transforms the relatively low inductive reactance of the pickup (which is about ohms) to a high impedance at the grid of the oscillator 57. This action does not change the percentage variation of impedance provided by the pickup. As in previously known systems, variation of the impedance in the grid circuit of the oscillator 51 varies the plate current of the oscillator, thereby providing a variable voltage drop in a load resistor 59 in the output circuit of the oscillator 51, which voltage drop conforms to the motion of the flexible stylus bar 21 and the movable inductor 2 in response to signal forces applied to the needle 31 by a record R.

From the foregoing description, it will be ap parent to those skilled in the art that I have provided a novel pickup device which has the many advantages set forth above. Although I have shown and described only a single, specific form of pickup device, it will be apparent that many other forms thereof, as well as variations in the particular one described, are possibly within the scope of my invention. It will also be apparent that, in place of the tuned plate, tuned grid oscillator circuit of Figure '7, other suitable reproducing circuits may be employed with my novel pickup device. For example, separate detector or discriminator circuits may be employed, or the pickup may be used to control feedthrough from an oscillator to a detector circuit. In view of the foregoing, I desire that the above description shall not be taken as limiting but only as illustrative.

I claim as my invention:

1. A signal translating device comprising a plurality of serially connected conductors arranged in mutually inductive relation with each other and each being constituted by a straight conductor, two of said conductors being stationary, and a third of said conductors being mounted for vibratory movement relative to each of said two conductors, said three conductors being so arranged that, upon movement of said third conductor, the mutual inductances between it and each of said two conductors will vary simultaneously in opposite sense.

2. A signal translating device comprising a plurality of serially connected conductors disposed in mutuallyinductive and laterally spaced relation with each other and each being constituted by a straight conductor, two of said conductors being stationary, and a third of said conductors being disposed between said two conductors and mounted for vibratory movement relative to each of said two conductors, said three conductors being thereby so arranged that, upon movement iii) of said third conductor, the mutual inductances between it and each of said two conductors will vary simultaneously in opposite sense.

3. A phonograph pickup device comprising a supporting member, a pair of stationary straight conductors supported thereon, a third straight conductor movably carried by said supporting member, said three conductors being arranged in laterally spaced substantially parallel inductive relation to each other, and said third conductor being disposed between and mounted for movement relative to said pair of conductors for varying the mutual inductanoes between it and each of said pair of conductors, and a phonograph needle element coupled to said movable conductor for deriving forces for moving said movable conductor relative to said stationary conductors.

4. A phonograph pickup device comprising a supporting member, a pair of stationary, straight conductors mounted on said supporting member in fixed inductive relation to each other and con nected so that the mutual inductance between said stationary conductors is in opposition to the self inductances thereof, a third, straight conductor disposed between said pair of stationary conductors in inductive relation to each of them, a vibratory member carried by said supporting member and movable thereon in response to external forces, said third conductor being secured at one end thereof to said vibratory member, and an anchoring member carried by said supporting member, said third conductor being connected at its other end to said anchoring mom-- her.

5. A phonograph pickup device comprising a supporting member, a pair of stationary, straight conductors mounted on said supporting member in fixed inductive relation to each other, a tenuous element providing a third straight conductor disposed. between said pair of stationary conductors in inductive relation to each of them, a vibratory member carried by said supporting member and movable thereon in response to ex ternal forces, said third conductor being secured at one end thereof to said vibratory member, an anchoring member carried by said supporting member, said third conductor being connected at its other end to said anchoring member, and said anchoring member being adjustable for ap plying more or less tension to said tenuous element and said third conductor.

HENRY S. H'UFF.

REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,305,626 Lee Dec. 22, 1942 2,334,178 Dodge Nov. 16, 1943 2,371,373 Eadmaieii Mar. 13, 1945 2,417,188 Slark Mar. 11, 1947 2,436,946 'Iatro Mar. 2, 1948 FOREIGN PATENTS Number Country Date 116,481 Australia Jan. 20, 1943 

